Photon detector

ABSTRACT

A photon detection system including a photon detector configured to detect single photons, a signal divider to divide the output signal of the photon detector into a first part and a second part, wherein the first part is substantially identical to the second part, a delay mechanism to delay the second part with respect to the first part, and a combiner to combine the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal.

The present invention relates to the field of photon detectors andmethods for detecting photons which are configured to detect thepresence of a single photon.

Detectors which are capable of detecting a single photon so-calledsingle photon detectors are an important component of any system whichoperates using the principles of quantum cryptography. Such systems relyupon the transmission of data bits as single particles, in this case,photons, which are indivisible.

Single photon detection is also useful as a low level light detectionmeans for time of flight ranging experiments, spectroscopy, medicalimaging or astronomy. Both in medical and astronomical applications thehigh energy photons (X-ray etc) or high energy particles are convertedin scintillators into many (10-100) low energy photons. These low energyphotons are then detected by avalanche photodiodes or photomultipliertubes. As the low energy, photons that are produced are scattered inspace there is a need for large area detectors which are very sensitive.Also arrays of such detectors allow the spatial distribution of lowenergy photons to be obtained in order to gain information about theoriginal photon.

One problem which many single photon detectors suffer from is that thesignal which is outputted due the detection of a single photon is oftenweak and sometimes difficult to distinguish from other artefacts of thedetector output.

For example, a particularly popular type of single photon detector isthe avalanche photo-diode (APD) operating in gated mode. In gated mode,to detect a photon, a short duration of high reverse bias is appliedacross the APD which is above the breakdown voltage of the diode. Anabsorbed photon generates an electron-hole pair in the APD, which uponseparation can trigger an avalanche of excess carriers. This avalancheof excess carriers causes a macroscopic and detectable current flowthorough the APD.

Although this macroscopic current is detectable, it is usually buriedwithin artefacts of the output signal of the APD caused by thecapacitance of the APD in reaction to the biasing gates. One solution tothis problem is to bias the APD to an extent where the avalanche currentdominates the output of the detector. However, this has the disadvantagethat the APD needs to be operated at relatively low frequencies.

Other ways to address this problem have been to mimic the response ofthe APD by a capacitor or by a second APD (Tomita et al. Optics Letters27 1827 to 1829 (2002). Both of these systems suffer with problems inaccurately reproducing the output of the first APD and in practicalitiesof combining the output of the first APD with that of the capacitor orsecond APD.

The present invention attempts to address the above problems and in afirst aspect provides a photon detection system comprising a photondetector configured to detect single photons, a signal divider to dividethe output signal of the photon detector into a first part and a secondpart, where the first part is substantially identical to the secondpart, delay means for delaying the second part with respect to the firstpart and a combiner for combining the first and delayed second parts ofthe signal such that the delayed second part is used to cancel periodicvariations in the first part of the output signal.

By dividing the output of the detector into two parts and combining thesignal from a period with the signal from later periods, periodicvariations in the output of the detector are removed. Since the detectoroutput is used to correct itself, the problems of the prior art whichtry to replicate the detector output are avoided.

Preferably, the photon detector is an avalanche photodiode, but may alsobe other photon detectors.

Preferably, the detector receives a periodic signal and said delay meansare configured to delay the second part of the signal by an integermultiple of said period. However, it is possible for the system toisolate a single period or a mulitple of periods from the second part ofthe signal and repeatedly use this isolated signal to cancel periodicvariations in the first part of the signal. For example, the period ofthe waveform may be stored digitally, the stored waveform may then beused to perform cancelling using digital processing.

Preferably, the system comprises means to apply a periodic gating signalto said detector. The gating signal may be a square wave signal or asinusoidal signal etc.

As the present invention enhances the presence of the signal due to asingle photon without increasing the bias required across the detector,the detector of the present invention may operate at higher frequenciesthan those of the prior art. The present invention may therefore have agating signal with a frequency of 50 MHz or more, preferably 70 MHz ormore, even more preferably 100 MHz or more.

As the present invention can be used at higher gating frequencies, itcan achieve quasi-continuous operation. In quasi-continuous operation,there is no synchronization required between the source of photons andthe detector. Quasi continuous operation is possible because for veryhigh gating frequencies, the period at which the detector is incapableof detecting photons is not prohibitive to the overall detectionefficiency.

To enhance quasi continuous operation, it is possible to vary the periodof the gating signal to broaden the detection window. The period may bevaried randomly or as noise.

Generally, the two signals which arrive at the combiner will bebalanced. However, the system may further comprise means to balance theamplitudes of the two signals arriving at the combiner. For example, thesystem may further comprise a tunable attenuator.

The system may also comprise means to invert one part of the signal withrespect to the other part. Inversion may be achieved at either thedivider, combiner or during transfer between the divider and combiner.Inversion may be achieved by many methods, for example, using a hybridjunction which performs division/combination and inversion.

It is also possible to use a differential component, for example, adifferential amplifier, to combine the signals.

In a second aspect, the present invention provides a quantumcommunication system comprising a sender and a receiver, said sendercomprising a source of pulsed radiation and an encoder for encodinginformation on said radiation pulses, said receiver comprising adetection system according to the first aspect of the present invention.

The quantum communication system may be configured for QKD protocols,for example said encoding means may be configured to encodes pulsesusing an encoding base randomly selected from two or more non-orthogonalbasis.

Synchronised operation is preferred for all QKD applications, as thesource always emits in regular interval. However, some protocols, forexample, DPSK and Stucki protocols as described below require coherencefor neighbouring pulses, which usually requires source operating at GHzfrequencies. The high speed operation of the detection system of thepresent invention is particularly advantageous for these protocols.

The present invention is of particular use in the so called one-way weakcoherent protocol (D Stucki et al, Applied Physics Letters 87,194108(2005)). Thus, in a further embodiment, the sender may comprises acoherent source and said encoder encodes information by selectivelytransmitting pulses having an average intensity of less than one photonand empty pulses have no photons.

Also, the high speed operation of the detection system mayadvantageously be used in the differential phase shift protocol (Takesueet al. New Journal of Physics 7 (2005) 232) Thus, in a yet furtherembodiment, the sender may comprise a coherent source, said encodingmeans being configured to encode photons with a first phase or a secondphase and the difference between the first phase and the second phase is180° and wherein the receiver is configured to interfere photons fromone pulse with those of the following pulse.

In a third aspect, the present invention provides a conditioning circuitfor conditioning the output of a photon detector, the circuitcomprising, a signal divider to divide the output signal of the photondetector into a first part and a second part, where the first part issubstantially identical to the second part, delay means for delaying thesecond part with respect to the first part and a combiner for combiningthe first and delayed second parts of the signal such that the delayedsecond part is used to cancel periodic variations in the first part ofthe output signal.

Although the detection system according to a first aspect of the presentinvention can be operated in a quasi-continuous mode, it may also begated and synchronised with the output of a periodic source. In a fourthaspect, the present invention provides a system comprising a periodicsource and a detection system according to the first aspect of theinvention, said detection system further comprising a gating signal,said gating signal being configured to gate said detector to detectduring the arrival times of the highest intensity signal from thesource.

The above system may be advantageously used for example in: time offlight ranging experiments to determine the distances of objects;industrial inspection as the ability to detect single photons allowsaccurate measurements to be made of the dimensions of an object bymeasuring a pulse reflected from an object under inspection; and timeresolved emission experiments to determine the chemical recipe of acomposite.

In a fifth aspect, the present invention provides a photon detectionmethod comprising:

-   -   providing a photon detector configured to detect single photons;    -   dividing the output signal of said photon detector into a first        part and a second part, where the first part is substantially        identical to the second part;    -   delaying the second part with respect to the first part; and    -   combining the first and delayed second parts of the signal such        that the delayed second part is used to cancel periodic        variations in the first part of the output signal.

The present invention will now be described with reference to thefollowing non-limiting embodiments in which:

FIG. 1 a is a schematic of a prior art detection system using anavalanche photodiode (APD), FIG. 1 b is a schematic plot of voltageagainst time for the input signal of the APD of FIG. 1 a and FIG. 1 c isa plot of the output signal as voltage against time for the detectionsystem of FIG. 1 a;

FIG. 2 is a detection system incorporating an avalanche photodiode and acapacitor in accordance with the prior art;

FIG. 3 is a schematic of a detection system according to the prior artcomprising two APDs arranged to cancel one another;

FIG. 4 a is a schematic of a detection system comprising an avalanchephotodiode in accordance with an embodiment of the present invention,FIG. 4 b is a plot of the input signal to the device of FIG. 4 a, FIG. 4c is a plot of a first part derived from the output signal of the APD ofFIG. 4 a, FIG. 4 d is a plot of the second part derived from the outputsignal of the APD of FIG. 4 a which has been delayed and FIG. 4 e is aplot of the self-differenced output signal produced by the device ofFIG. 4 a;

FIG. 5 a is a variation on the device of FIG. 4 a, FIG. 5 b is a firstpart of the output signal of the device of FIG. 5 a, FIG. 5 c is adelayed second part of the output signal of the APD in FIG. 5 a and FIG.5 d is a plot of the output signal of the detection system of FIG. 5 a;

FIG. 6 a is a schematic of a further variation on the detection systemof FIG. 4 a and FIG. 6 b is a plot of the input signal to the detectionsystem of FIG. 6 a;

FIG. 7 is a schematic of a detection system which is a variation of thedetection system of FIG. 4 a;

FIG. 8 is a plot of the dark count probability against photon detectionefficiency for the detection system of FIG. 4 a;

FIG. 9 is a plot of the count rate against photon flux per secondshowing the quasi-continuous detection operation of the presentinvention;

FIG. 10 a is a quantum communication system having a detection system aspreviously described with reference to FIG. 4 a, FIG. 10 b is theclocking signal for the quantum communication system of FIG. 10 a, FIG.10 c shows the outputted laser pulse for the transmitter system of FIG.10 a, FIG. 10 d is a plot of the signal arriving at the detector for thedetection system of FIG. 10 a and FIG. 10 e shows a potential gatingsystem for the detector used in FIG. 10 a;

FIG. 11 a is a schematic of a quantum communication system which uses aprotocol based on phase coherence between adjacent pulses, FIG. 11 b isa plot of the pulse sequence which may be sent using the system of FIG.11 a and FIG. 11 c is a schematic of the gating requirements of thedetectors of FIG. 11 a; and

FIG. 12 a is a schematic of a quantum communication system which uses adetector in accordance with the present invention and which is based ona differential phase shift protocol, FIG. 12 b is a schematic of thepulse train sent from the transmitter to the detector; FIG. 12 c is aschematic of the pulse train which passes through the short arm of thereceiving interferometer, FIG. 12 d is a schematic of the pulse trainwhich passes through the long arm of the receiving interferometer, FIG.12 e is a schematic of the possible photon detection times by detectorD1 of FIG. 12 a and FIG. 12 f is a schematic of the possible photondetection time s by the detector D2 of FIG. 12 a.

FIG. 1 a is a schematic of a known detection system which may be used todetect single photons. It comprises an avalanche photodiode 1 and aresistor 3. The avalanche photodiode (APD) 1 is configured in reversebias. An input signal which acts as a gating signal is shown in FIG. 1 bis applied between input 5 and ground 7.

The input voltage shown in FIG. 1 b is a periodic series of rectangularvoltage pulses, which varies between a first value V₁ and a second valueV₀. V₁ is selected to be above the breakdown voltage V_(BR) of theavalanche photodiode 1. When such a voltage is applied to the avalanchephotodiode, the detector becomes sensitive to incoming photons. Anabsorbed photon generates an electron-hole pair in the APD, which areseparated by the electric field inside the APD. Due to the high electricfield within the avalanche region of the APD, the electron or hole maytrigger an avalanche of excess carriers causing a macroscopic anddetectable current flow thorough the APD.

The macroscopic current is usually detected by monitoring the voltagedrop across a resistor 3 as shown in FIG. 1 c. A voltage spike 13indicates that a photon has been detected. However, as the APD has afinite capacitance, typically one pico-Farad, the output also contains acharging pulse 9 due to the charging of the APD capacitance whenreacting to the rise edge of the gating pulse, followed by a dischargingdip 11 due to the discharging of the APD capacitance when reacting tothe falling bias at the falling edge of a gate pulse. The charging pulseis positive, and often obscures a photon induced avalanche.

It is clear from the results shown in FIG. 1 c that it is difficult toisolate peak 13 which is due to absorption of a single photon. For anavalanche due to photon absorption to be detectable, one method is toincrease the APD bias voltage so that amplitude of an avalanche spikeexceeds that of the charging pulse. An avalanche can then be detected bysetting discrimination level above that of all charging pulses. However,such a method has serious shortcoming. APDs usually containcrystallographic defects, and those defects act as traps to confineelectrons from the macroscopic avalanche current flow during a detectionevent. Trapped electrons will be released spontaneously after some delaycausing a second, spurious avalanche when the following gate is applied.Such a spurious pulse is called “afterpulse”, and its probabilitydepends on the size of the avalanche current. To limit the afterpulseprobability, APDs are typically operated at low gating frequencies (upto a few MHz) if biasing is used to achieve avalanches larger thancharging pulses.

To increase the APD operation frequency, it is necessary to remove orlimit the amplitude of the charging pulse 9.

One attempt at doing this is shown in FIG. 2. To avoid unnecessaryrepetition, like reference numerals will be used to denote like featureswith those of FIG. 1. FIG. 2 again has an avalanche photodiode 1 andresistor 3. A capacitor 21 and further resistor 23 are formed in serieswith the avalanche photodiode 1 and resistor 3 such that resistors 3 and23 are connected back to back.

From this circuit, the combined DC and pulsed bias for the APD varyingbetween V₀ and V₁ is applied to the APD, while just the pulse signal(varying between V₁ and V₀) is applied to the capacitor. The outputsignal from the capacitor 21 will be similar to the output signal fromAPD 1 in the absence of absorption of a photon. The output from the APD1 and capacitor 21 are then combined in hybrid junction 25. Hybridjunction 25 will reverse the phase of one of its two inputs. Therefore,the hybrid junction 25 combines the output signals from both the APD 1and the capacitor 21 with a 180° phase difference so that they nearlycancel. This allows the charging 9 and discharging 11 peaks to bepartially cancelled.

The arrangement of FIG. 2 suffers from the problems in that it isdifficult for the capacitor to simulate the response of the APD. Also,there is a need from the pulsed output signal from both the APD 1 andthe capacitor 21 to arrive at the hybrid junction 25 at exactly the sametime. Further, it is also necessary for the output of the APD 1 and theoutput of a capacitor 21 to have equal amplitudes.

FIG. 3 illustrates a further improvement to the system of FIG. 2 wherecapacitor 21 is now replaced with a second APD 23.

To avoid unnecessary repetition, like reference numerals will be used todenote like features.

A second resistor 31 and a second APD 33 are provided in the samepositions as second resistor 23 and capacitor 21 of FIG. 2. The outputof the first APD 1 and second APD 33 are then combined at hybridjunction 35 with 180° phase difference in order for the components tocancel one another. The output of the hybrid junction is then fed intodiscriminator 37 (not shown) which in turn determine their presence ofpeaks due to single photon detection.

The device of FIG. 3 suffers from many of the problems of the systemshown in FIG. 2. Furthermore, selecting APDs with similarcharacteristics involves costly characterisation of devices. Inparticular, it is very difficult to find two APDs with identicalelectrical characteristics even with careful selection of devices. TheAPDs are likely to have different capacitances, which will mean that thecancellation of their output signals will be poor.

FIG. 4 a schematically illustrates a system in accordance with anembodiment of the present invention. As before, the device comprises anavalanche photodiode provided in series with a resistor 53.

An input signal as shown in FIG. 4 b may be applied. This is identicalto the input signal described with reference to FIG. 1 b. The voltagedropped across the resistor 53 is first input to power splitter 55.Power splitter 55 divides the output signal into a first part as shownin FIG. 4 c and a second part which is identical to the first part shownin FIG. 4 c. These two signals are then output via ports 57 and 59 ofpower splitter 55. The signal which is output via port 59 enters a delayline 56 which serves to delay the signal by a duration equal to thegating period. The delayed signal is shown in FIG. 4 d. The first partof the signal and the delayed second part are then fed into hybridjunction 61. Hybrid junction 61 combines the first and the delayedsecond parts of the signals with 180° phase shift to give the outputshown in FIG. 4 e.

As can be seen in FIG. 4 c, the detection of a photon by APD 51 producesan avalanche signal 73. This avalanche signal is then repeated oneperiod later in the trace of FIG. 4 d. By combining 4 c and 4 d, a peak77 and a dip 75 are seen in the trace of FIG. 4 e which indicates thepresence of a photon. The provision of a positive peak followed by anegative dip (or a negative dip followed by a positive peak dependent onthe configuration of the equipment) allows a clear signature indicatingthe detection of a photon.

A preferred mode of detection is to test for both the presence ofpositive peak 77 and negative dip 75 in the output signal.Alternatively, however it may be more convenient to test for just thepositive peak alone or just the negative dip alone. The peaks or dipsmay be detected using discrimination techniques. Discriminationtechniques use a discriminator level. Voltage signals which are largerthan said discriminator level are assumed to be due to detection of aphoton.

The power splitter 55 may be of the type which is sold under part numberZFRSC-42+ from Mini-circuits and the hybrid junction is also availableunder part number ZFSCJ-2-4 which is also available from Mini-circuits.The exact delay can be realised by using two co-axial cables with twodifferent lengths that link the power splitter and the hybrid junction.It should be noted that the combination of the power splitter 55, thedelay line 56 and the hybrid junction 61 may be integrated onto a singleprinted circuit board.

FIG. 5 shows a variation on the device described with reference to FIG.4. The device of FIG. 5 a takes the output from an APD and resistor (notshown) and provides it to hybrid junction 81. Hybrid junction 81 splitsthe output into a first part and a second part is described withreference to the power splitter 55 of FIG. 4 a. However, hybrid junction81 also introduces a 180° phase shift between the first part and thesecond part of the signal. The first part of the signal is output viaoutput 83 and the second part is sent via output 85 into delay line 87.

The first part of the signal is shown in FIG. 5 b and the second part ofthe signal which has passed through delay line 87 and is identical toFIG. 5 b except being inverted and delayed by a clock period is shown asFIG. 5 c.

The two signals shown in FIGS. 5 b and 5 c are then combined to producethe output signal of FIG. 5 d. The signals are combined during powercombiner 89 which does not need to allow a phase shift to one of thesignals since this has been already performed by hybrid junction 81.

The systems of FIG. 4 a and FIG. 5 a have both used combinations ofpower splitters/combiners and hybrid junctions. However, the hybridjunction may be replaced by a combination of phase shifters and powercombiners. For example, a power combiner and a 180° phase shifter, ortwo power combiner/splitters with a 90° phase shift etc.

In a further variation on the systems of FIG. 4 a and FIG. 5 a, atuneable RF attenuator is provided which may be used in either of theinputs to hybrid junction 61 (FIG. 4 a) or power combiner 89 (FIG. 5 a)to ensure that the two signals reach the hybrid junction or powercombiner with equal amplitudes.

Typically, all the hybrid junctions and power splitter/combiners havefinite response frequency range. For example, hybrid junction,Mini-circuits ZFSCJ-2-4 has a frequency range of 50 MHz to 1 GHz. It maynot work well when signal contains frequency components outside of therange, and the cancellation may not be perfect. To improve signal tonoise ratio, further bandpass filters may be used to filter out thosefrequency components. In FIG. 4 a, for example, a low bandpass filtermay be placed after the hybrid junction output.

FIG. 6 shows a further variation on the system described with referenceto FIGS. 4 and 5.

The system of FIG. 6 a has an avalanche photodiode 51 and a resistor 53as described with reference to FIG. 4 a. Further, the voltage droppedacross the resistor 53 is taken to power splitter 55 which splits thesignal into a first part and a second part. The first part beingoutputted via output 57 and the second part via output 59 into delayline 56. The first part of the signal and the delayed second part arethen fed into hybrid junction 61 which combines the two parts of thesignal with 180° phase difference.

However, in the apparatus of FIG. 6 a, the input voltage signal is asinusoidal voltage signal as shown in FIG. 6 b and not the periodictrain of rectangular pulses as shown in FIG. 1 b. It is possible to biasthe detection system of FIG. 6A with a sinusoidal signal as long as thesignal has sufficient voltage swing to bias the APD above and below thethreshold for avalanche breakdown. In fact, the detector may be biasedwith any periodic voltage signal.

In response to the sinusoidal gating voltages, the APD output is alsosinusoidal. Superimposed on the sinusoidal output are occasionalavalanche spikes due to photon detection. The amplitude of avalanchespikes is typically much smaller than that of the sinusoidal output.However, as described previously with reference to FIG. 4 a, by using apower splitter, delay line, and hybrid junction, the sinusoidalcomponents can be largely cancelled and the avalanche spikes becomeclearly visible.

Furthermore, any small remaining components of the sinusoidal signal maybe removed from the output of the hybrid junction 61 by a band rejectionfilter 63 which is tuned to the frequency of the sinusoidal signal. Thesignal is passed to amplifier 65 and then discriminator 67 in order todetermine the presence or absence of photon induced spikes in theoutputted signal.

FIG. 7 shows a further variation in the system of FIG. 4, theconfiguration is the same as that described with reference to FIGS. 4 a,5 a and 6 a. The output signal is then fed into power splitter 101.Power splitter 101 divides the signal into a first part and a secondpart. The first part is output via output 103 and the second part viaoutput 105 which is further fed into delay line 107. The two parts ofthe signal are then fed into differential amplifier 109. Due to theconfiguration of amplifier 109, only the difference of the two inputs isamplified. The output signal is not shown but will be similar to thatshown in FIG. 4 e.

FIG. 8 is a plot of the output of the detection system of FIG. 4 a usingan APD driven with a square wave operating at 625.1 MHz. The dark countprobability per gate is plotted against photon detection efficiency. TheAPD is cooled at −40° C. To measure the detection efficiency, the APDwas illuminated with laser pulses with a wavelength of 1550 nm, aduration of 100 ps, a repetition rate of 9.76 MHz, and an intensity of0.1 photons per pulse on average.

In the measurement, the output of hybrid junction 61 (FIG. 4 a) was fedinto first a broadband amplifier, and then a discriminator whichidentifies each individual avalanche spike and converts the spike into aTTL pulse output. The TTL pulses are then counted by a pulse counter.The APD gating signal is square wave with 8-V amplitude, superimposed ona DC bias voltage typically 2 V below the avalanche breakdown voltage ofthe APD (which is 47 V). The points in FIG. 8 were recorded by varyingthe DC bias level. The higher the DC bias, the higher the detectionefficiency. However, the detector dark count probability of thedetector, defined as the output probability in the absence of any inputlight, also increases with the DC bias.

The results from FIG. 8 are surprisingly good since the APD is beingdriven with a frequency 625.1 MHz. This is considerably higher than theusual gating frequencies for APDs which are typically a few MHz. At thesame time, little deterioration has been found in the efficiency of thesame device and dark count probability as compared to low frequencyoperations. The ability to drive an APD at this frequency is possibledue to self-differencing technique described in the present invention.

The high gating frequency allowed by the present invention means thatthe detection system can operate quasi-continuously. In quasi-continuousoperation, there is no synchronization required between the source ofphotons and the detector. Quasi continuous operation is possible becausefor very high gating frequencies, the period at which the detector isincapable of detecting photons because its bias is below the breakdownvoltage of the APD, is not prohibitive to the overall detectionefficiency.

In order to avoid any unintentioned synchronisation of the source andthe detector it is desirable to vary the frequency of the signal used togate the APD, for example the drive frequency may be varied randomly byapplying some noise to the frequency.

In such a scheme, the delay line which introduces a time delay betweenthe first part and the second part of the signal remains constant.However, the gating frequency may vary by a small amount, for example,50 kHz, which essentially broadens the time window over which thedetector is capable of detecting single photons.

FIG. 9 shows a plot of detector operated in quasi continuous mode usingsimilar biasing condition described with reference to FIG. 8. Themeasured count rate i.e. the number of photons received per second ismeasured as a function of the photon flux. The photons were emitted by acw laser diode operating at 1550 nm. The emission time of each photon isnon-deterministic, and the APD is no longer able to synchronise with theincoming photons. The measurement result therefore indicates theperformance of the detector used in continuous mode.

The actual measured count rate is shown with the black squares. Thetriangles show the net photon count rate which is the measured photoncount rate minus the dark count rate. The dark count is determined bymeasuring the output of the detector in the absence of illumination.

It can be seen from the graph of FIG. 9 that high photon count rates areachieved close to 10 MHz, which is limited mainly by electronicsdeadtime. The detector shows a linear response over a 40 dB range, and adetection efficiency of 1.2%.

The detection system described in FIGS. 4 to 9 can be used in quantumcryptography systems, for example, the system of FIG. 10.

In FIG. 10A, a sender (Alice) 201 sends photons to a receiver (Bob) 203.The sender and receiver are linked by an optical fibre 205.

Alice generates single photons, which she encodes and sends to Bob,along with a bright laser pulse to act as a clock signal.

Alice's equipment comprises a single photon source 207. The singlephoton source is made from a pulsed laser diode 209 and an attenuator211. The laser produces a single optical pulse for each clock signalwith a repetition period of T_(clock). Typically each laser pulse has aduration of d_(laser)=50 ps. The level of attenuation is set so that theaverage number of photons per pulse which are sent by Alice are muchless than 1 (μ<<1), for example μ=0.1 is typical. Alternatively, thelevel, of attenuation may be varied from pulse to pulse as described inGB2404103.

A clock signal is provided to the laser 209 by bias electronics 210. Thebias electronics may comprise a timing unit, a driver for the signallaser 209, a driver for the clock laser 227 which will be describedlater and a driver for the phase modulator 223 which will be laterdescribed.

The photon pulses from the photon source 207 are then fed into animbalanced Mach-Zender interferometer 213. The interferometer 213consists of an entrance fibre coupler 215, a long Arm 217 with a delayloop of fibre 219 designed to cause an optical delay, a short arm 221with a phase modulator 223, and an exit fibre coupler 225 which combinesthe fibres 217 and 221 from the long and short arms respectively. Thelength difference of long and short arms corresponds to an opticalpropagation delay of t_(delay). Typically the length of the delay loop219 is chosen to produce a t_(delay)˜0.5 ns. A photon travelling throughthe long arm will lag that travelling through the short arm 221 by atime of t_(delay) at the exit of the interferometer 213.

The output of Alice's interferometer 213 is multiplexed with the outputfrom a bright clock laser 227 at a WDM coupler 229. The clock laser 227operates under the control of the biasing circuit 210. The clock laser227 may emit at a different wavelength from that of the signal laser209, so as to facilitate their easy separation at Bob's 203 end. Forexample the signal laser 209 may operate at 1.3 μm and the clock laser227 at 1.55 μm or vice versa.

The multiplexed signal and clock pulses are transmitted to the recipientBob 203 along optical fibre link 205.

Bob's equipment 203 is similar to Alice's equipment 201. Bob's equipment203 comprises a WDM coupler 231 which is used to de-multiplex the signalreceived from Alice 201 into a signal from Alice's bright clock laser227 and the pulses from Alice's signal laser 209.

The bright clock laser 227 signal is routed to an optical receiver 233to recover the clock signal for Bob to synchronise with Alice. Theoptical receiver 233 transfers this signal to biasing circuit 255.Biasing circuit 255 synchronises various parts of Bob's equipment 203.

The signal pulses are fed into a polarisation controller 235 to restoretheir original polarisation.

The signal pulses then pass Bob's interferometer 237. Bob'sinterferometer 237 is similar to Alice's interferometer and has a longArm 239 which comprises an optical fibre delay loop 241 and a variablefibre delay line 243. The short arm 45 of the interferometer 39comprises a phase modulator 247. Phase modulator 247 is controlled bybiasing circuit 255 in accordance with the signal received from clocklaser 227.

The long arm 239 and the short arm 245 of the interferometer areconnected to a 50/50 fibre coupler 249 with a single photon detector 251and 253 attached to each output arm of the fibre coupler 249. The singlephoton detector 251 attached to one arm of the coupler 249 will bereferred to as detector A and the single photon detector 253 attached tothe other arm of the output coupler 249 will be referred to as detectorB. Photon detectors 251 and 253 are controlled by biasing circuit 255 inaccordance with the signal received from clock laser 227.

The variable delay line 243 at Bob's interferometer is adjusted to makethe optical delay between its two arms 239 and 245 identical as thatbetween the arms of Alice's interferometer 213, t_(delay).

There are four possible paths for a signal pulse travelling from Alice'ssignal laser 209 to Bob's single photon detectors 251 and 253:

-   -   i) Alice's Long Arm 217-Bob's Long Arm 239 (Long-Long);    -   ii) Alice's Short Ann 221-Bob's Long Ann 239 (Short-Long);    -   iii) Alice's Long Arm 219-Bob's Short Arm 245 (Long-Short); and    -   iv) Alice's Short Ann 221-Bob's short arm 245 (Short-Short).

Bob's interferometer 237 is balanced by adjusting the variable delay 243so that photons taking paths (ii) and (iii) arrive at nearly the sametime, at the exit coupler 249 of Bob's interferometer. Nearly the sametime means within the signal laser coherence time which is typically afew picoseconds for a semiconductor distributed feed back (DFB) laserdiode.

FIG. 10 b is a trace of the clock which is output from laser 209 to thereceiver 203. Typically the clock signal has a repetition of 1 GHz. FIG.10 c is a trace of the laser pulse which is used to generate the signalpulses.

FIG. 10 d is a plot of the optical signal seen by the detectors 251 and253 of receiver 203. Photons taking paths (ii) and (iii) corresponds tothe large central peak in FIG. 10 d. Photons taking path (i) have apositive delay t_(delay) (later arrival time), and those taking path(iv) have a negative delay t_(delay) (early arrival time) compared topaths (ii) and (iii). These form the smaller satellite peaks of FIG. 10d. Only photons arriving in the central peak shown in FIG. 10 d undergointerference and are encoded by both Alice and Bob. Thus only thesephotons are of interest.

FIG. 10 e is a plot of the gating bias which is in synchronization withthe clock bias shown in FIG. 10 b. Bob gates his detectors 251, 253 torecord only photons in the central peak and not those in the earlier orlater satellite peak.

By controlling the voltages applied to their phase modulators 223, 247,Alice and Bob determine in tandem whether paths (ii) and (iii) undergoconstructive or destructive interference at detectors A and B, 251, 253.

The variable delay 243 can be set such that there is constructiveinterference at detector A 251 (and thus destructive interference atdetector B 253) for zero phase difference between Alice and Bob's phasemodulators. Thus for zero phase difference between Alice's and Bob'smodulators and for a perfect interferometer with 100% visibility, therewill be a negligible count rate at detector B 253 and a finite countrate at A 251.

If, on the other hand, the phase difference between Alice and Bob'smodulators is 180°, there should be destructive interference at detectorA 251 (and thus negligible count rate) and constructive at detector B253. For any other phase difference between their two modulators, therewill be a finite probability that a photon may output at detector A 251or detector B.

In the four-state protocol, which is sometimes referred to as BB84,Alice sets the voltage on her phase modulator to one of four differentvalues, corresponding to phase shifts of 0°, 90°, 180°, and 270°. Phase0° and 180° are associated with bits 0 and 1 in a first encoding basis,while 90° and 270° are associated with 0 and 1 in a second encodingbasis. The second encoding basis is chosen to be non-orthogonal to thefirst. The phase shift is chosen at random for each signal pulse andAlice records the phase shift applied for each clock cycle.

Meanwhile Bob randomly varies the voltage applied to his phase modulatorbetween two values corresponding to 0° and 90°. This amounts toselecting between the first and second measurement bases, respectively.Bob records the phase shift applied and the measurement result (i.ephoton at detector A 251, photon at detector B 253, photon at detector A251 and detector B 253, or no photon detected) for each clock cycle.

In the BB84 protocol, Alice and Bob can form a shared key bycommunicating on a classical channel after Bob's measurements have takenplace. Bob tells Alice in which clock cycles he measured a photon andwhich measurement basis he used, but not the result of the measurement.Alice then tells Bob the clock cycles in which she used the sameencoding basis and they agree to keep only those results, as in thiscase Bob will have made deterministic measurements upon the encodedphotons. This is followed by error correction, to remove any errors intheir shared key, and privacy amplification to exclude any informationknown to an eavesdropper.

Detectors 251 and 253 of receiver 203 in FIG. 10 a may conveniently beprovided by the detection systems discussed with reference to FIGS. 4 to9.

FIG. 10 a quantum communication system which may be used for thewell-known BB84 protocol or B92 protocol. Recently, other quantumcommunication systems have been developed which use different protocols.The detector of the present invention with its higher frequency gatedoperation or quasi-continuous operation is particularly advantageous forthese new types of quantum communication systems.

FIG. 11 shows quantum communication apparatus for another type ofone-way quantum key distribution scheme. A sender's apparatus 301comprises a coherent laser 303 which outputs a continuous pulsed beaminto intensity modulator 305. Intensity modulator 305 either transmitsthe pulse or blocks it almost completely. The output from the intensitymodulator 305 is then passed into attenuator 307 which attenuates thebeam so that each pulse contains less than 1 photon on average. Theoutput is then passed along fibre 309 to receiver 311.

At receiver 311, the beam first encounters beam splitter 315. Beamsplitter 315 is configured to pass most of photons along path 317 andthe remainder along path 319 into interferometer 320.

Path 317 extends to qubit detector 320 which is a detection system asdescribed with reference to any of FIGS. 4 to 9. Path 319 directsphotons to 50/50 beam splitter 321 which sends photons either along longarm 323 or short arm 325. Photons which pass along long arm 323encounter phase shifter 327. The photons from path 323 and 325 are thencombined by a beam splitter 329 which outputs photons either intodetector 331 or into detector 333 dependent on the phase correlationbetween the photons.

FIG. 11 b shows how bits may be encoded by the sender 301. Each bitconsists of two “pulses”. Bit 0 is encoded by a pulse sequence whichcomprises a first pulse which has intensity μ and a second pulse withintensity 0 and bit 1 is encoded with a first pulse which has intensity0 and a second pulse which has intensity μ. In addition to sending bit 0or bit 1, decoy states are sent which comprise two pulses with intensityμ.

Due to the coherence of the laser 303, there is a well defined phaserelationship between any two neighbouring non-empty pulses. Therefore,within each decoy sequence, there is coherence. There is also coherencebetween some of the sequences, for example in the case where bit 0 isfollowed by bit 1. If an eavesdropper intercepts the pulses, thecoherence of adjacent non-empty pulses will be affected. This loss ofcoherence can be determined by interferometer 320.

The interferometer is configured (by applying an appropriate phase shiftand an appropriate difference between the length of the long 323 andshort 325 arms) to ensure that photons exit into just one of the twodetectors 331 and 333 when interference takes place. Thus by monitoringthe count rate in the other detector it is possible to detecteavesdropping. Detectors 331 and 333 are described with reference toFIGS. 4 to 9.

Alice sends a stream of pulses as shown in FIG. 11 b. The receiver 302will then inform Alice 301 for which pulses the qubit detector 320 isfired. Alice will then advise Bob which bits should be thrown away asthey are due to decoy states. It should be noted that Bob only informsAlice in which sequence a count was received and does not inform Alicewhether these bits were measured as bit 0 or bit 1.

The detectors 331 and 333 are used to monitor coherence. When twoadjacent non-empty pulses pass through the interferometer 322,interference will take place between the early pulse passing thoroughthe long arm 323 and the late pulse passing through the short arm 325.Interference determines that there is a finite probability for detector331 to fire, but negligible probability for detector 333 to fire, atthis particular detection time bin. Violation of this means the loss ofcoherence. In the QKD, the receiver tells the transmitter when and whichof the detectors 331 and 333 fires, and this allows the sender toestablish whether or not coherence was broken.

The sender and receiver will then run error correction and privacyamplification on the results dependent on the loss of coherence which iswell-known in the art to determine the secret key.

The detection systems described in FIGS. 4 to 9 can conveniently be usedwith the communication system of FIG. 11 since they provide higher speedoperation and can also work in CW mode. FIG. 11 c shows the gatingvoltage which is supplied to the APD's of the detection system. Thegating pulse is synchronised with the expected arrival time of eachincoming pulse. This gating system would be applied to the threedetectors of the receiver 302 in FIG. 11 a.

FIG. 12 shows a further quantum communication system. The communicationsystem of FIG. 12 a also uses coherence between adjacent pulses in orderto communicate a key. The sender 401 comprises a pulsed coherent laser403 which outputs to a phase modulator 405. The phase modulator israndomly varied to apply a phase modulation of either 0 or 180 degrees.It is possible to apply some other phase shifts providing that thedifference between the two phase shifts is 180. The signal is thenattenuated by attenuator 407 which ensures that there is less than onephoton per pulse. This is then transmitted down fibre 409 to receiver411. Receiver 411 comprises an interferometer 413. Interferometer 413comprises a first beam splitter 415 which directs photons either downshort arm 417 or long arm 419. Long arm 419 comprises a phase shifter421. The long arm 419 and the short arm 417 are recombined at secondbeam splitter 423 which then outputs to a first detector 425 and asecond detector 427.

The sender 401 sends a pulse train where each pulse is modulated byeither a 0° phase shift or a phase shift of 180 degrees. This is shownin FIG. 12 b. At the receiver's side, the received pulse train is splitinto two paths: the short arm 417 and the long arm 419. FIG. 12 c showsthe pulse train at the short arm 417, and FIG. 12 d shows the pulsetrain at the long arm 419. Notice that the pulse train in long arm isdelayed by exactly a clock period. The interferometer 413 is configuredby varying the phase shift or the relative length or arms 417 and 419 toessentially introduce a 1 bit delay. Therefore, the interferometer 413can be thought of as interfering the pulse train shown in FIG. 12 c withthe pulse train shown in FIG. 12 d. The pulse train in FIG. 12 d is thesame as that of FIG. 12 c but has a 1 bit delay. This results in thedetection of pulses shown in FIG. 12 e at detector 425 and the detectionof pulses shown in FIG. 12 f at detector 427.

The security relies upon the uncertainty in the detection time of aphoton. Due to coherence, the photon's wavefunction is spread over anumber of adjacent time bins. An eavesdropper eve's detection of aphoton causes its wavefunction to collapse into a single time bin, andshe is only able to know the phase difference between two particulartime bins. Therefore she is not able to re-produce the original statewhich is coherent over a number of time bins. This preventsintercept-and-resend attack. It is also inherently secure against photonnumber splitting attack. Eve may split a photon from the pulse train,however, it will not necessarily collapse into the same time bin as thatdetected by Bob.

Previously, the differential phase shift QKD is implemented using a CWdetector, and discrimination between neighbouring time bins is done inthe post processing with a time-resolution of 10-ps. Such apost-processing requires either sophisticated hardware or software dataprocessing. With detectors running in gated mode, time discrimination isautomatic requiring no post processing.

The invention claimed is:
 1. A photon detection system comprising: aphoton detector configured to detect single photons, the photon detectorbeing configured to generate an output signal comprising voltage spikesgenerated in response to single photons and a periodic component,wherein the voltage spikes are superimposed on the periodic component; avoltage source configured to apply a gating signal to the detector asthe periodic component, the gating signal comprising a voltage signalwhich swings above and below a breakdown voltage of the photon detector;a signal divider to divide the output signal of the photon detector intoa first part and a second part, wherein the first part is substantiallyidentical to the second part; a delay line configured to delay thesecond part with respect to the first part by an amount selectedaccording to the period of the gating signal; a combiner for combiningthe first and delayed second parts of the signal to obtain a combinedsignal such that the delayed second part is used to cancel the periodiccomponent in the first part of the output signal; and a discriminatorconfigured to compare the combined signal with a discriminator level andto determine the detection of a photon in response to a peak and/or adip in the combined signal.
 2. A system according to claim 1, whereinthe photon detector is an avalanche photodiode.
 3. A system according toclaim 2, wherein said delay line is configured to delay the second partof the signal by an integer multiple of said period.
 4. A systemaccording to claim 3, wherein the gating signal is a square wave signal.5. A system according to claim 3, wherein the gating signal has afrequency in excess of 50 MHz.
 6. A system according to claim 3, whereinthe period of the gating signal is varied.
 7. A system according toclaim 1, further comprising an attenuator to balance the amplitudes ofthe two signals arriving at the combiner.
 8. A system according to claim1, further comprising means to invert one part of the signal withrespect to the other part of the signal.
 9. A photon detection systemaccording to claim 1 wherein the discriminator is configured todetermine detection of a photon in response to the presence of both apeak and a dip in the combined signal.
 10. A system comprising aperiodic source and a detection system according to claim 1, saiddetection system further comprising a gating signal, said gating signalbeing configured to gate said detector to detect during the arrivaltimes of the highest intensity signal from the source.
 11. A quantumcommunication system comprising: a sender and a receiver, said sendercomprising a source of pulsed radiation and an encoder for encodinginformation on said radiation pulses, said receiver comprising adetection system according to claim
 1. 12. A quantum communicationsystem according to claim 11, wherein said encoder encodes pulses usingan encoding base randomly selected from two or more non-orthogonalbasis.
 13. A quantum communication system according to claim 11, whereinsaid sender comprises a coherent source and said encoder encodesinformation by selectively transmitting pulses having an averageintensity of less than one photon and empty pulses have no photons. 14.A quantum communication system according to claim 11, wherein saidsender comprises a coherent source and said encoder is configured toencode photons with a first phase or a second phase and the differencebetween the first phase and the second phase is 180°, and wherein thereceiver is configured to interfere photons from one pulse with those ofthe following pulse.
 15. A photon detection method comprising: providinga photon detector configured to detect single photons, the photondetector being configured to generate an output signal comprisingvoltage spikes generated in response to single photons and a periodiccomponent, wherein the voltage spikes are superimposed on the periodcomponent; applying a gating signal to the detector as the periodiccomponent, the gating signal comprising a voltage signal which swingsabove and below a breakdown voltage of the photon detector; dividing theoutput signal of said photon detector into a first part and a secondpart, wherein the first part is substantially identical to the secondpart; delaying the second part with respect to the first part by anamount selected according to the period of the gating signal; combiningthe first and delayed second parts of the signal to obtain a combinedsignal such that the delayed second part is used to cancel the periodiccomponent in the first part of the output signal; comparing the combinedsignal with a discriminator level and determining the detection of aphoton in response to a peak and/or a dip in the combined signal.
 16. Aphoton detection method according to claim 15, wherein delaying thesecond part with respect to the first part comprises to delaying thesecond part of the signal by an integer multiple of said period.
 17. Aphoton detection method according to claim 15, wherein the period of thegating signal is varied.
 18. A photon detection method according toclaim 15, further comprising inverting one part of the signal withrespect to the other part of the signal prior to combining the two partsof the signal.